I've always hated the idea promoted by some AI researchers that "intelligence is just prediction." Specifically in the context of your challenge, since a brain must decide the next word before uttering it, is it not vacuously true that any conceivable method of producing speech is "predicting" it? You're asking for evidence that it isn't; I'm asking for evidence that question has any meaning.
Sure, any production in that case can be classified as prediction. What I’m saying is that such distinction, if at all possible, is far from self-evident.
